This is understandable, due to Frodo carrying the One Ring from the Shire to Mordor for so long. I'm guessing this is the after-thoughts, somewhere during those four years after his journey to the Shire. I think after so many years, I did finally understand the reason why Frodo chose to leave with the Ring-bearers on the last ship to leave Middle-earth. I guess since reading this story, I am still right, but there's more reason behind why Frodo left.